1. Field of the Invention
This invention relates to multipath delay estimation in wireless communication systems and, in particular, to a more robust and flexible method and system of estimating multipath delays.
2. Description of the Related Art
In code division multiple access (CDMA) and wideband CDMA (WCDMA) mobile communication systems, such as the Universal Mobile Telecommunication System (UMTS), data is transmitted using a spread spectrum modulation technique wherein the data is scattered across a wide range of frequencies. Each channel is assigned a unique spreading code that is used to spread the data across the frequency range. The spreading code is called a pseudo-random noise (PN) code and is composed of a binary sequence of 1's and 0's (or 1's and −1's), called “chips,” that are distributed in a pseudo-random manner and have noise-like properties. The number of chips used to spread one data bit, or chips/bit, may vary and depends, in part, on the data rate of the traffic channel and the chip rate of the system. To recover the transmitted data, the received signal must be despread with the same spreading code using the same chip rate. Furthermore, the timing of the demodulation must be synchronized, that is, the despreading code must be applied to the received signal at the correct instant in time.
Achieving the proper timing can be difficult due to multipath fading effects where the same transmitted signal travels along multiple paths to arrive at the receiver unit at different times. Referring to FIG. 1, for example, the receiver unit 100 may receive the transmitted signal from a base station 102 on a direct and unobstructed propagation path (Path 1). However, many other propagation paths (e.g., Path 2, Path 3) also exist because, in most cases, the transmit antenna of the base station 102 is not focused narrowly enough in any given direction. Thus, multiple instances of the same signal may be received by the receiver unit 100 at different times as portions of signal are reflected off various objects and obstacles (e.g., a house 104, a building 106) in the surroundings before arriving at the receiver unit 100. In the reverse direction, transmission from the receiver unit 100 to the base station 102 may experience similar multipath fading effects.
Most CDMA based systems use RAKE receivers that are capable of identifying and tracking the various multipath signals for a given channel. Multipath signals with similar propagation distances may then be combined, depending on the time resolution of the transmission system and the instantaneous phase relationship of the multipath signals, to form a distinct multipath component. Each multipath component is assigned a despreader (RAKE finger) that has a copy of the spreading code, but which copy has been delayed in time relative to the spreading code used for the direct path component. The amount of delay time in the despreader is set to match the path delay of the corresponding multipath component. After despreading, the multipath components from the various despreaders are coherently combined to produce an estimate of the data or symbols being transmitted.
For the above arrangement to be effective, the RAKE receiver requires up-to-date knowledge of the multipath delays of the channel. This knowledge is important in order to maximize the signal-to-interference ratio of the detected multipath signal. In addition, the smaller the number of paths available at the receiver unit, the larger the probability that the detected paths may experience simultaneous deep fade. The utilization of diversity, or lack thereof, may lead to serious and often catastrophic degradation of the block error rate (BLER).
One way to identify the multipath signals is to search for the multiple paths over a range of possible despreading delays. This path searching can be obtained by transmitting a pilot signal from the base station and applying a series of predefined despreading delays at the receiver unit. Where the predefined delays happen to coincide with the arrival times of the multipath signals, a larger-magnitude channel estimate will result. The resulting delay profile, which can be a complex delay profile (CDP) or a power delay profile (PDP), may then be subjected to peak detection, and the location of the peaks are reported to the RAKE receiver as estimates of the multipath delays of the channel.
FIG. 2 illustrates an exemplary PDP of a given channel for one pass or iteration of the path search. The vertical axis in FIG. 2 represents the magnitude of the detected signal, while the horizontal axis represents the size of the delays applied. The PDP of FIG. 2 shows all the signals that are received by the receiver unit, including noise and interference signals. However, only the peaks in the PDP correspond to the multipath signals of the channel, which peaks together form the impulse response of the channel. In this iteration, the search window (or delay spread) includes a total of X number of delay units. One delay unit may be, for example, 0.1 μs, and k delay units is simply k times one delay unit. In subsequent iterations or passes, the search window may be adjusted both in position (i.e., starting time) and size (i.e., number of delay units) in order to continually update the RAKE receiver with the most recent multipath delay estimates.
However, the processing and power consumption expenses of frequently executing this path searching routine is usually prohibitive. Therefore, typical delay estimations use shortened search windows, reduced searcher resolution, and additional, short sub-searcher routines to produce higher resolution estimates of certain areas of the PDP, for example, the M delays indicated in FIG. 2. Even with these reduction measures, it has been found that the task of properly scheduling the searcher passes and positioning the search window can still pose serious difficulties under many channel conditions. Consequently, some multipath components escape detection, thereby degrading both the instantaneous SIR (signal-to-interference ratio) and the utilized diversity in the multipath fading environment.
Since the realization of the delay estimation function in RAKE receivers depends on the specific system parameters and hardware resources, it is difficult to present a universally “best” solution that may be applied to all systems. For example, although there exist a number of basic architectures for delay estimation, there are even more numerous detailed variations thereof. Nevertheless, a fairly advanced and practical implementation of a delay estimator can be said to include the following stages: path searcher, tuning fingers, path resolution and tracking, and searcher window placement and scheduling
FIG. 3 illustrates a basic delay estimator in a RAKE receiver. As can be seen, the delay estimator 300 includes a path searcher (PS) 302, tuning fingers (TF) 304, a path resolution and tracking module 306, and a searcher window placement and scheduling module 308, all interconnected as shown. The path searcher 302 is a device that computes instantaneous channel impulse response estimates (complex or power) over a range of delays that constitutes a significant fraction of the maximum delay spread allowed by the system. The CDP or PDP for a given delay value is estimated by correlating the received data for the pilot symbols with an appropriately delayed copy of the spreading sequence, a method which is well known in the art. Often, the path searcher 302 is used mainly as a means to detect the existence of paths and, therefore, its output resolution may be somewhat lower than the resolution required by the RAKE receiver.
Tuning fingers 304 are devices for producing a high-resolution instantaneous CDP or PDP over a narrow delay window, for example, the M delays in FIG. 2. The tuning fingers 304 may include M despreaders, each despreader tuned to one of the M neighboring delays. The despreaders of the tuning fingers 304 are similar to the despreaders of the RAKE fingers of the path searcher except they are usually more closely spaced together. Because of the high resolution, tuning fingers 304 are commonly used to locally refine the coarser PDP information provided by the path searchers 302.
The path resolution and tracking module 306 includes a set of signal processing and logical algorithms that extract the physical path location information from the path searcher 302 and the tuning fingers 304 outputs. This location information is then presented as delay estimates to subsequent RAKE receiver stages, and assignment of distinct paths to RAKE fingers is made. Once assigned, the assignments remain constant over a significant time to allow reliable power and interference estimation. The degree of complexity of the signal processing and logical algorithms varies significantly depending on system parameters, and may range from simple peak detection to sophisticated deconvolution and filtering algorithms.
The scheduling and window placement module 308 includes control logic that determines the timing of the path searcher and tuning finger activation and their respective window positions for each path searcher cycle. The timing of the activation may be fixed (periodic), or it may depend on signals derived from the surrounding environment, while the positioning usually depends on the location of previously detected paths.
The nature of the delay estimates provided by the delay estimator 300 shown in FIG. 3 is intrinsically instantaneous in that, for a simple implementation, a particular step can be performed based only on the information provided by the previous stage. However, it is often found that such memoryless operations do not yield satisfactory results under certain demanding channel conditions. In low signal-to-interference ratio (SIR) conditions, for example, the peaks in the PDP due to the physical paths are difficult to distinguish instantaneously. Fading effects also make it difficult to detect and track paths based merely on the instantaneous path magnitude at the time of the path searcher pass. Further, for channels with wide delay spread, the precise path searcher window placement is critical to avoid missing paths with significant energy.
One way to alleviate the above concerns is to build some temporal averaging (filtering) into certain points of the path detection and control logic stages (e.g., the path resolution and tracking stage). It has been found, though, that successful performance of the delay estimator at the system level is quite sensitive to finding the proper combinations of filtering parameters. These parameters, in turn, often depend on a particular targeted channel condition. Accordingly, it is desirable to provide a more robust delay estimator architecture that is less susceptible to the difficult channel conditions mentioned above.